The present invention relates to a thermal type flow measurement apparatus, more particularly, to a thermal type flow measurement apparatus having an asymmetrical passage for measuring a fluid flow rate, and measuring the flow rate by a way of sensing a temperature difference of an upstream side thermal sensitive element and a downstream side thermal sensitive element of the heating element.
The following thermal type flow measurement apparatus is known. That is, it has a heating element and thermal sensitive resistors disposed, in a direction of fluid flow, on both the upstream and the downstream sides of the heating element. A fluid flow rate is measured based of the difference between a temperature sensed by the upstream side thermal sensitive resistor and a temperature sensed by the downstream side thermal sensitive resistor.
In the thermal type flow measurement apparatus using such a temperature difference sensing method, when the fluid flow rate to be measured becomes large, the temperature of the upstream side thermal sensitive resistor does not decrease below a certain value, and the temperature of the downstream thermal sensitive resistor tends to decrease.
In such a case, as the fluid flow rate increases, the difference between the temperatures sensed by the upstream side thermal sensitive resistor and the downstream side thermal sensitive resistor, ideally, ought to become larger. Since, however, such a relation ship is not obtained, the flow measurement sensitivity decreases in a large flow rate range.
Therefore, in a technical matter described in Japanese Patent Laid-Open No. 2003-65820 (Patent Document 1), an electric power supplied to a heating resistor is controlled so that the average of the temperatures sensed by the thermal sensitive resistors on the upstream and the downstream sides, respectively, is kept constant.
In such a prior art, when, in a large flow rate range, the temperatures sensed by the thermal sensitive resistors on the upstream and the downstream sides both start decreasing, the electric power supplied to the heating element is controlled so as to increase the temperatures, thereby making it possible to obtain a temperature difference corresponding to the flow rate.
In measuring an intake air rate for an automobile engine, there are cases in which not only a forward flow (a fluid flow in a direction to cause a thermal type flow measurement apparatus to output a positive value, i.e., a flow of air toward the engine), but also a reverse flow (fluid flow in a direction to cause the thermal type flow measurement apparatus to output a negative value, i.e., a flow due to spitting from the engine) occurs.
In a case where the thermal measurement apparatus is a household gas flow meter, no reverse flow occurs, so that no problem associated with the flow direction is caused. But, in a case where an amount of air taken into an automobile engine is to be measured, a reverse flow of a non-negligible magnitude is caused by spitting from the engine.
The flow measurement apparatus is therefore required to have adequate sensitivity not only for a forward flow but also for a reverse flow.
In a case where an amount of air taken into, for example, an automobile engine is to be measured, dust if adhering to a flow measurement element causes the sensing accuracy to decrease. There is a technique devised to prevent dust having entered a sub-passage (:secondary passage; bypass passage) from reaching the flow measurement element. It uses a subpassage which is, for example, scroll-shaped making a fluid passage asymmetric with respect to the flow direction.
When such a fluid passage asymmetric with respect to the flow direction is used, a forward fluid flow and a reverse fluid flow show largely different, asymmetric characteristics.
Conventional arts do not at all take into account improvement in reverse flow sensing sensitivity realized by using a fluid passage asymmetric with respect to the flow direction.
Even if the technique matter described in the Patent Document 1 is applied to a case where a fluid passage asymmetric with respect to the flow direction is used, flow sensing errors result with no measures taken in terms of reverse flow sensing.